Implement AlertDetector background task that runs every minute to detect
and create alerts for various anomalies:
- Rate limit detection: monitors masters hitting rate limits
- Error spike detection: flags keys with >= 10% error rate
- Quota exceeded: warns when key quota usage >= 90%
- Provider down: alerts when API keys have >= 50% failure rate
Includes fingerprint-based deduplication with 5-minute cooldown to
prevent duplicate alerts for the same issue.
